Michelle Barger

Doctoral Dissertation Announcement

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Candidate: Michelle Barger

Degree of: Doctor of Philosophy

Department: Geosciences

Title: The Influence of Organic Acids on Uraninite Dissolution and Uranyl Sorption on Kaolinite

Committee:
Dr. Carla M. Koretsky, Chair
Dr. Johnson R. Haas
Dr. Alan E. Kehew
Dr. Patricia A. Maurice

Date: Friday, October 14, 2011 11:00 a.m. to 1:00 p.m.
1120 Rood Hall

Abstract:
Within anoxic near surface aqueous settings, where UO2(s) may be released, an opportunity to encounter and complex with organic acids may occur. Reactions between UO2(s) and ligands may promote the solubility and mobility of uranium. Organic ligands investigated in the dissolution work include citric acid, NTA and EDTA. Exposure to the ligands had little effect on UO2(s) solubility. The log activity of UO2(s) hydrolysis under reduced conditions was -6.56. Under all measured conditions of ligand concentration, the data consistently show an increase in uranium concentration to a median log U activity of -4.89. The observed solubility of UO2(s) in the presence of chelating organic ligands is inconsistent with expected values based on literature data, possibly due to inefficient controls on oxidation. Although the solubilities values are higher than expected, these experiments suggest that strong chelating agents will have little effect on UO2(s) solubility.
The second part of this study concerned U(VI) sorption onto Georgia kaolinite. Sorption was investigated as a function of ionic strength and pCO2 concentration in the presence or absence of EDTA, citrate and fulvic acid. U(VI) sorption on kaolinite does not depend strongly on ionic strength, but a strong dependence on pCO2 is observed, with less sorption of U(VI) occurring with increasing pCO2. U(VI) sorption is enhanced by the addition of EDTA, citric acid or fulvic acid at low pH, likely due to formation of ternary surface complexes. At high pH, U(VI) sorption decreases, presumably due to formation of competitive aqueous organic ligand-U(VI) complexes. Surface complexation of U(VI) as a function of ionic strength and pCO2 is well described using a nonelectrostatic or a double diffuse layer model. However, neither model correctly simulates all of the U(VI) sorption edges in the presence of ligands, possibly due to incorrect estimates of CO2-3(aq) concentrations, inaccuracies in the predicted aqueous U(VI)-ligand speciation, partial dissolution of the clay by the organic ligands, or due to poor representation of the electrical double layer in the presence of the organic ligands.